(1) Field of the Invention
The present invention relates to a nonwoven sheet composed of polyethylene terephthalate filaments. More particularly, the present invention relates to a nonwoven sheet having an improved heat deterioration resistance and a high elongation and also to a nonwoven sheet having specific properties, and prepared from the above-mentioned nonwoven sheet.
(2) Description of the Prior Art
A highly molecularly oriented, highly crystalline, drawn polyethylene terephthalate filament has a good heat resistance and a good dimension stability, and therefore, filaments of this type are widely used for clothing and industrial materials. However, since the filaments have a low elongation at break point, they cannot be used in fields where a post processing such as molding is required.
Undrawn polyethylene terephthalate filaments have a high elongation at break point and a high heat shrinkability are known, and these filaments can be subjected to a post processing such as molding in the form of nonwoven sheets. Accordingly, these filaments can be applied in various fields. However, these undrawn polyethylene terephthalate filaments are subject to heat deterioration, in that their elongation at break point is reduced when they are heated.
Accordingly, although the commercial possibilities for undrawn polyethylene terephthalate filaments are broad, they are used only in limited specialized fields. For example, undrawn polyethylene terephthalate filaments are used as binder filaments for nonwoven sheets by utilizing the low softening point thereof (see Japanese Examined Patent Publication (Kokoku) No. 48-41115 and Japanese Unexamined Patent Publication (Kokai) No. 57-139554), or undrawn polyethylene terephthalate filaments are used for obtaining nonwoven sheets having an improved drape characteristic by preparing a nonwoven sheet while mixing drawn filaments with the undrawn filaments and drawing the nonwoven sheet by utilizing the high elongation of the undrawn filaments in the nonwoven sheet (see Japanese Examined Patent Publication No. 45-6296).
Under the above-mentioned background, we carried out research into the micro-structures of polyethylene terephthalate filaments with a view to improving the heat deterioration resistance, and as a result, found that the heat deterioration resistance can be improved if the outer layer portion of the section of a single filament is more highly oriented and has a higher degree of crystallization than the central portion. It also was found that if a nonwoven sheet composed of undrawn polyethylene terephthalate filaments having a thus-improved heat deterioration resistance is subjected to an appropriate post processing treatment, the resulting nonwoven sheet composed of undrawn polyethylene terephthalate filaments can be applied to uses not heretofore expected. The present invention was completed based on these findings.